Effect of alkyl substituents on the thermodynamics of the self-association of purine in aqueous solution

Abstract

The thermodynamic parameters for the self-association of several alkyl-substituted purines have been determined by applying the isodesmic model to the molar enthalpies of infinite dilution at different initial concentrations. Carbon-bonded alkyl groups have been shown to increase the tendency to association by making the association enthalpy more negative. In contrast, the increasing size of the N9-bonded alkyl groups appears to reduce the negative entropy contribution and thus stabilize the associated complexes. These findings are discussed in terms of dipole–induced-dipole interactions and hydrophobic bonding.